(a) Field of the Invention
The present invention relates to a liquid crystal display (LCD) device. More specifically, the present invention relates to an active matrix type liquid crystal panel and a liquid crystal display device adapted to improve an aperture ratio.
(b) Description of the Related Art
Liquid crystal display devices have the advantages in that they are thin and light and that they can be driven at low voltages and have low power consumption. Accordingly, liquid crystal display devices are used in various kinds of electronic devices such as televisions, desktop personal computers (PCs), notebook PCs, personal digital assistants (PDAs), mobile phones, and the like. In particular, active matrix liquid crystal display devices, in which a thin film transistor (TFT) as a switching element is provided for each picture element, exhibit excellent display characteristics, which are comparable to those of cathode ray tube (CRT) displays, because of high driving capabilities thereof, and therefore they have been widely used even in fields where CRT displays have been used heretofore, such as televisions and desktop PCs.
A typical liquid crystal display device has a structure in which liquid crystals are contained between two transparent substrates made of glass plates. On one substrate, a TFT, a picture element electrode, and the like, are formed for each picture element. On the other substrate, color filters, a common electrode (counter electrode), and the like, which face the picture element electrodes, are formed. In the description below, for convenience, the substrate on which the TFTs, the picture element electrodes, and the like, are formed is referred to as a “TFT substrate,” and the substrate on which the color filters, the common electrode, and the like, are formed is referred to as a “counter substrate.” Further, the structure including the TFT substrate, the counter substrate, and the liquid crystals contained therebetween is referred to as a “liquid crystal panel.”
FIG. 1 schematically shows, in the form of a plan view, the constitution of a TFT substrate in a prior art active matrix liquid crystal panel.
As shown in the drawing, a plurality of gate bus lines 11 extending horizontally (laterally), a plurality of data bus lines 12 extending vertically (longitudinally), and a plurality of auxiliary capacitance (Cs) bus lines 13 extending parallel to the gate bus lines 11, are formed on the TFT substrate 10. The gate bus lines 11 and the Cs bus lines 13 are provided with the respective equal pitches (wiring intervals), and the data bus lines 12 are also provided with equal pitches (note, approximately ⅓ of the pitches of the gate bus lines 11 and the Cs bus lines 13). Thus, each region surrounded by two adjacent gate bus lines 11 and two adjacent data bus lines 12 constitutes a unit picture element. In the region of this unit picture element, a picture element electrode 14 (indicated by a dashed line) is formed. Different regions (picture element electrodes 14) of unit picture elements are provided for the colors of R (red), G (green), and B (blue), respectively. Three laterally adjacent R, G, and B sub-pixels constitute one picture element (pixel). Each of R, G, and B picture elements (sub-pixels) has a rectangular shape with an aspect ratio of approximately 3:1. It is noted that reference numeral 15 (portion surrounded by a dotted line) denotes two thin film transistors (TFTs) connected in series, reference numeral 16 denotes a contact hole for connecting the drain region of the TFTs 15 to the data bus line 12, reference numerals 17 and 18 denote contact holes for connecting the source region of the TFTs 15 to the picture element electrode 14, and reference numeral 19 denotes a semiconductor region forming an auxiliary capacitance Cs together with the Cs bus line 13 for each picture element.
As shown in FIG. 1, in the constitution of the prior art liquid crystal panel (TFT substrate 10), each of the R, G, and B picture elements (sub-pixels) constituting one picture element (pixel) has a rectangular shape with an aspect ratio of approximately 3:1. Accordingly, in order to reduce the area of a portion of the relevant picture element which does not effectively contribute to display, to a minimum (i.e., reduce a decrease in the aperture ratio to a minimum), it has been necessary to minimize the wiring length of the Cs bus line 13 crossing the relevant picture element region. For this purpose, the Cs bus lines 13 have needed to be provided parallel to the gate bus lines 11. In such a layout, the picture element regions (picture element electrodes 14) are necessarily defined by the data bus lines 12 as boundaries. Taking into consideration the overlaps with the picture element electrodes 14 and the distances between adjacent picture element electrodes 14, the wiring width of the data bus line 12 cannot be made too narrow. Namely, since the wiring width of the data bus line 12 needs to be appropriately made wide, it has caused a decrease in the aperture ratio.
Moreover, as the trend toward higher definition grows, a reduction in the time for writing into picture elements has posed a problem. For example, in the case where the frame frequency is 60 Hz (the number of frames scanned for one second is 60), the scanning time for one frame is approximately 16.7 ms (= 1/60 s). In the case where the pixel format is VGA (Video Graphics Array: 640×480 picture elements), the time assigned for one horizontal line is approximately 32 μs. If the number of picture elements further increases (SVGA (Super VGA: 800×600 picture elements), XGA (eXtended GA: 1024×768 picture elements), or the like), the time assigned for one horizontal line is further reduced, and the time for writing into picture elements needs to be reduced correspondingly. In order to reduce this write time, for example, it can be considered that the transistor size of each picture element is increased. However, where the transistor size is increased, the area of a portion of the relevant picture element which does not effectively contribute to display increases. Accordingly, the aperture ratio decreases, and there occurs a problem in that display becomes dim.
Moreover, in a field sequential color system in which data is divided by time division for each of the colors of R, G, and B, or the like and written into picture elements and in which R, G, and B light sources or the like are time-sequentially turned on synchronously with the writing, there has occurred a problem in that the time assigned for one horizontal line is further reduced. For example, in a general active matrix LCD, in order to prevent flickers, a write operation is performed at approximately 60 Hz ( 1/60 s per one frame). In the case where field sequential drive is performed by constituting a color image for one frame using three fields of the three primary colors of RGB, a period for one field is 1/180 s (= 1/60 s×⅓), and the time assigned for one horizontal line needs to be reduced by a factor of approximately ⅓. Namely, there has been a disadvantage in that the time allowable for writing is limited.
Furthermore, in the field sequential color system, after data (e.g., R data for one field) is written into all of the picture elements constituting one frame, the color of a light source needs to be switched (e.g., a light source of G or B needs to be turned on). Accordingly, the total time required for writing data of all the colors (R, G, and B) into all the picture elements becomes longer by an amount corresponding to the above-described switching. In particular, there has been a disadvantage in that the time for writing into all the picture elements increases as the number of picture elements increases.
Moreover, in an impulse drive system in which a light source is turned on in a pulsating manner (turned on only for a partial time of one-frame time) in order to improve the image quality of a moving video picture, the total time required for writing into all the picture elements also needs to be reduced. However, similar to the case of the above-described field sequential drive, there has been a disadvantage in that the time for writing into all the picture elements increases as the number of picture elements increases.
As the art relating to the above-described prior art, for example, as described in Japanese unexamined Patent Publication (JPP) 10-232408, there is a liquid crystal panel in which picture element electrodes and TFTs are formed to correspond to the intersections of control bus lines and data bus lines, in which capacitor bus lines forming auxiliary capacitances together with the picture element electrodes are formed, and which has auxiliary capacitance patterns branching from the capacitor bus lines and extending along the data bus lines. In this liquid crystal panel, where a short circuit between a data bus line and an auxiliary capacitance pattern is found out, the defective portion can be easily recovered. Further, as the art relating to the aforementioned field sequential drive, for example, as described in JPP 2002-311411, there is a technology in which a high-definition liquid crystal panel is realized using an amorphous silicon-type liquid crystal element and in which uniform back lighting is realized by adopting a point light source type as a back light necessary for the liquid crystal panel.
As described above, in the prior art, each of R, G, and B picture elements (sub-pixels) constituting one picture element (pixel) has a rectangular shape with an aspect ratio of approximately 3:1. This requires that the Cs bus lines 13 be provided parallel to the gate bus lines 11, and that the wiring width of the data bus line 12 be made wide accordingly. Consequently, there has been a problem in that the aperture ratio decreases. Further, with the development of the trend toward higher definition, there has been a problem in that a sufficient write time cannot be ensured for liquid crystal panels of drive systems in which the time allowable for writing into picture elements is limited, and for liquid crystal panels having a large number of picture elements.